Apart from actions against wrong treatment (e.g. reverse polarity, DE.sub.-- 3408657), several methods and circuits are proposed for the protection of accumulators from damage by overcharging.
It is well-known from literature (e.g. ISBN 3-7883-0142-2) that the break of the energy supply at predefined values (e.g. end-charging-voltage, current, temperature or -gradient, amount of charge, a.s.o.) is not a reliable criterion. Therefore it is proposed to use a certain feature in the trend of charging current or -voltage.
For example, the voltage trend of an empty cell shows a decrease during constant current charging (CCC) at the start, an increase until full charge, and thereafter again a decrease. A characteristic maximum appears. The final decrease is caused by overtemperature and pressure and may not exist in open cells. The (current-depending) voltage has reached a final value, which will not rise further (maximum), even under continuous energy supply.
Similar statements concerning the occurence of an extreme at full charge apply for the trend of the (inner) resistance of the cell and for the trend of the charging current at constant voltage charging (CVC).
At CCC it is known to break the charging current when the accumulator voltage has decreased for a certain degree (-dV method), or at CVC when the current, after having passed a minimum, is increasing (FR-A-1489957) to a predetermined level (U.S. Pat. No. 3,889,172).
A disadvantage of these methods is, that the accumulator will be overcharged and that the effect does not appear with open or leaking cells.
EP.sub.-- 0181112 discloses the turning point of the voltage curve to avoid overcharging in CCC. At this point the accumulator is not full-charged and the effect does not appear in a full-charged cell.
A charger for lead acid batteries is described in U.S. Pat. No. 4,710,694, which terminates the CCC at a predefined slope of voltage. An additional time limited charging step is needed to reach full charge. Because the slope depends on the number and capacity of the cells, this principle applies for semi-empty cells of a certain type and size only.
FR-A-2203198 discloses a charging method for lead acid batteries. The charging current is interrupted in intervals for a measurement cycle. Charging is terminated when the measured voltage is not higher than in the previous cycle. This feature characterizes the full cell, but it also occurs on empty accumulator or with changing temperature or current. Charging duration is significantly elongated by the interruptions of the charging cycle.
A further disadvantage of all previously described methods is the fact, that the criterion for terminating the charging summarizes different effects in one feature and cannot distinguish if they rely on the state of charge or are caused by other influences.